[PATCH 1/2] winemine: Rename difficulty to avoid translation issues.

[Julian Rüger]

Signed-off-by: Julian Rüger <jr98(a)gmx.net>
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I changed the strings and left IDM_ADVANCED and friends as is, to keep
it simple.

If you want me to change them as well, please say so. (Also in that
case, would IDM_INTERMED be acceptable to avoid changing the indentation
of a whole bunch of other defined constants?)